1. Field of the Invention
The present invention relates to electronic flash cameras, in general, and to an electrical circuit for controlling the firing duration of an electronic flash tube prior to and/or during an exposure interval, in particular.
2. Description of the Prior Art
Electrical circuits for controlling the firing duration of an electronic flash tube prior to and/or during an exposure interval are well known in the art. In, for example, U.S. Pat. No. 4,697,906 to Kobayashi et al., the firing duration of a flash tube in an electric flash device is controlled by a conventional NPN transistor in series with the flash tube. In one embodiment thereof, when this series transistor is placed in its ON or conductive state, it causes a previously charged main storage capacitor to become connected between an anode and a cathode of the flash tube. Placing the transistor in its ON state also causes a trigger signal to be applied to an electrode of the flash tube to thereby initiate flash tube firing. When the sensed scene light level reaches a predetermined level, a discharge cut-off signal is generated by a discharge cut-off signal generating circuit. The series transistor and a thyristor coupled thereto are turned OFF in response to this discharge cut-off signal which collectively shut-off the scene-illuminating flash tube. In addition to the size and complexity of this transistor/thyristor type flash tube control system, there is a substantial delay between the point in time when the flash tube is commanded to turn off and the point in time when it actually turns off in response to a turn-off command. This delay may cause excessive scene illumination if compensation is not provided. More importantly though, such a delay may prematurely deplete the charge in the main storage capacitor necessitating a delay in the flash tube firing cycle time so that the main storage capacitor in the electronic flash control circuit may be re-charged to the requisite charge level for proper flash tube firing.
An electrical circuit for controlling the firing duration of a flash tube with a minimum delay between the flash firing ON and OFF commands and the actual turning on and off of the flash tube, has been disclosed in U.S. Pat. No. 4,839,686 to Hosomizu et al. A control circuit for a flash tube is described therein which includes a main storage capacitor adapted to be charged from a suitable power source, a flash tube and an insulated gate bipolar transistor or IGBT, which can be activated between conductive and non-conductive states, that is connected in the discharge path of the main storage capacitor through the flash tube. A signal responsive to a manually generated voltage or flash firing command is applied to a gate terminal of the IGBT to activate same and thereby connect the main storage capacitor between an anode and a cathode of the flash tube. The flash tube control circuit further includes a trigger circuit portion that triggers the flash tube to its ON state in response to the IGBT being activated from its non-conductive to its conductive state. The light output of the flash tube is extinguished by a flash terminating command that removes the flash tube firing voltage, previously applied to the gate of the IGBT, in response to a signal representative of the magnitude of sensed scene light. By removing this voltage from the gate of the IGBT it changes from its conductive to its non-conductive state which interrupts light-producing current flow to the flash tube. In other words, light output from the flash tube is controlled by the magnitude and pulse width of a single voltage pulse that is applied to a gate of an IGBT. Advantages resulting from this type of flash tube control circuit are size, cost and simplicity of design. A significant disadvantage is the variation in the duration of the flash tube firing interval that results from variations in certain IGBT electrical characteristics, from one IGBT to another.
In flash tube control circuits where precise control of the flash tube firing interval is essential in order to produce a proper photographic exposure, variations in this interval between two identical flash tube control circuits employing the same type of IGBT would produce over or under exposed photographic images from one camera to another in electronic flash cameras employing the same flash tube control circuit as described above, for example, with respect to the Hosomizu et al. patent.
In, for example, U.S. Pat. No. 4,894,678 to Farrington et al., an exposure control system is described in which subject distance is derived by measuring subject reflectivity immediately prior to an exposure interval. Subject reflectivity is determined, in part, by illuminating a subject within a scene to be photographed with visible and infrared light for a fixed period of time and then integrating the infrared component of this light that is reflected from the scene subject. The final integrated value thereof constitutes a measure of subject reflectivity. In order to obtain the same measure of subject reflectivity from one electronic flash camera to another employing this same subject reflectivity measuring arrangement, it is essential that the same flash duration interval result when the flash tube is fired for subject reflectivity determining purposes.
In the flash tube control circuit described in the above-noted U.S. Patent to Hosomizu et al., an IGBT is employed to turn ON and turn OFF a scene-illuminating flash tube as previously explained. Activation and de-activation of the IGBT is achieved, in effect, by the application of a voltage to and the removal of a voltage from a gate of this device. The time required to activate the IGBT is determined, in large part, by the magnitude of the capacitance between the gate and an emitter thereof. The greater the magnitude of this capacitor the longer will be the time required to activate the IGBT from its non-conductive to its conductive state. As a general rule, the magnitude of this capacitance may vary over a wide range such as from 600 to 1200 picofarads, from one IGBT to another with a corresponding variation in IGBT turn-on time. Variations of this magnitude would introduce significant errors into reflectivity measurements and into subject distance measurements derived therefrom where the accuracy of such measurements are dependent upon having a known flash firing interval. The magnitude of gate to emitter capacitance can be controlled by selecting only those devices having the same such capacitance from one IGBT to another. However, the cost of this selection process both in terms of the testing that would be required and the number of IGBTs that would have to be discarded, would substantially increase the cost of all of the IGBTs meeting a particular capacitance requirement.